Spin crossover properties of Fe(III) complexes in [Fe (bzacen)(tvp)]BPh4·nSolv chain structures: EPR study

Article abstract of DOI:10.1002/mrc.5064

Two types of Fe(III) polynuclear iron(III) 1D-chain coordination compounds of the general formula [Fe (L)(tvp)]BPh₄ nSolv, where L = dianion of N,N′-ethylenebis (benzoylacetylacetone)2,2′-imine (bzacen), tvp = 1,2-di(4-pyridyl)ethylene were syn

Full text of DOI:10.1002/mrc.5064

Turanova Olga (Orcid ID: 0000-0003-1834-6096)
Spin Crossover Properties of Fe (III) Complexes in [Fe(bzacen)(tvp)]BPh4‧nSolv Chain
Structures. EPR study.
T. A. Ivanova1, I. V. Ovchinnikov1, O. A. Turanova1, L.V. Bazan1, V. A. Shustov1, R. G.
Batulin2, M. A. Cherosov2
1Zavoisky Physical-Technical Institute, FRC Kazan Scientific Center of RAS, Kazan, Russia
2Institute of Physics, Kazan Federal University, Kazan, Russia
Correspondence Olga Turanova, Kazan E. K. Zavoisky Physical -Technical Institute (
KPhTI )
Sibirsky tract, 10/7 420029 Kazan, Russia
e-mail: lightur@mail.ru
Abstract
Two types of Fe (III) polynuclear iron (III) 1D-chain coordination compounds of the
general formula [Fe(L)(tvp)]BPh4 nSolv, where
L
=
dianion of N,N’-
ethylenebis(benzoylacetylacetone)2,2’-imine (bzacen), tvp = 1,2-di(4-pyridyl)ethylene were
synthesized and studied by the EPR and magnetic susceptibility methods in the temperature
range (100-300) К. Two types of spin-variable complexes are formed depending on the time
of precipitation of the complexes from the same solution leading to differently solvated
species. They have different characteristics of the local ligand field and the spin transition
behavior. The thermodynamic parameters of spin transitions were determined from the
temperature dependence of the EPR signals integral intensity. The energy levels splitting
values obtained by analyzing g-factors of low-spin Fe (III) centers evidenced not only on the
crucial role of low-symmetry distortions on the principal possibility of spin-crossover
processes, but also on the temperature peculiarities of spin transitions.
KEYWORDS
EPR spectroscopy, Iron (III) complexes, spin-crossover transition, local ligand field
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INTRODUCTION
The synthesis of coordination compounds of Fe (III) with spin-crossover properties
and characteristics necessary for practical application is an actual problem. The search of a
relationship between the chemical structure and properties of the spin state of Fe (III)
complexes is most important for its decision. There are many factors which determine the
spin state of these Fe (III) complexes: the composition and ligand structure, the orientation of
the equatorial and axial ligands, the type of anion, the presence or absence of a solvate in the
complex and the nature of intermolecular interactions between the complexes.[1-6] The
research results show that each of these causes lead to the redistribution of electron density
on ligands and affects the strength and symmetry of the ligand field, which can lead to a
change of Fe (III) ions spin-variable properties. However, the conclusions made are actual
only for specific ligand systems, and their use requires a more detailed study of the changes
in the local ligand field at the Fe (III) ion. EPR spectroscopy provides detailed information on
the local symmetry and electronic structure of the spin-crossover Fe (III) complexes.[7–10]
The Fe (III) complexes with N4O2 coordination are of great interest because they are
potentially favored for manifestation of spin-variable properties.[11-13] The influence of the
configuration of the tetradentate equatorial N2O2 donating Schiff base ligand (L) in a series
of mononuclear Fe (III) complexes of the composition [FeLX2]Y with X = Him and Y =
BPh4 was studied in Ref..[1, 14, 15] The variation of L in the series salen - acen - bzacen,
happen, salacen led to a change in the spin state of the complexes: high spin (HS) - low spin
(LS) - spin transition. According to the authors, the spin state of the Fe (III) ion is determined
mainly by the strength of the ligand field of the N4О2 coordination site: salen provides a
weak ligand field, acen provides a strong ligand field, bzacen, happen ligands and an
asymmetric salacen ligand - an intermediate ligand field. At the same time, based on the
results of studying [Fe(X-SalEen)2]Y complexes by EPR and Mossbauer spectroscopy,
concluded that the asymmetry of the local ligand field on the Fe (III) ion also plays an
2
important role in the formation of a spin transition.[16] The large splitting of low-spin T2g
terms in this case lead to possibility of spin transition between the lower Kramers doublet of
this multiplet and the excited high-spin multiplet 6A1g.
Not only monomolecular Fe (III) complexes are important, but also polynuclear
systems that collectively respond to external influences. Schiff tetradentate N2O2 bases L
tend to form 1-D chains when the [Fe(III)(L)]+ fragment is combined with a bridged bidentate
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ligand. The interconnection between ligand conformation L and magnetic properties of
Fe (III) complexes in a 1-D polymeric materials of the general formula [Fe(III)L(X)]+, where
L = acen, salen, salphen, X = bidentate N - donating bridging ligand, were studied in by
methods of X-ray analysis and magnetic susceptibility.[17] In compounds with the bridging
ligand X = tvp and the equatorial ligand L = salen, which contains aromatic groups, Fe (III)
ions were in a high-spin state. Replacing the equatorial ligand with L = acen, in which only
aliphatic groups are present, led to a change of the spin state of Fe (III) ions to a low-spin
one; incomplete spin transition was observed only at temperatures above 250 K.
In our opinion, the use of equatorial ligands L = bzacen, happen, or salacen, combining
both aromatic and aliphatic fragments, is very likely to create favorable conditions for the
realization of the spin transition in this class of compounds.
In order to obtain a complex with spin-changing properties, the compound
[Fe(L)(tvp)]BPh4
with
the
equatorial
ligand
L
=
dianion
of
N,N’-
ethylenebis(benzoylacetylacetone)2,2’-imine (bzacen) and a bidentate ligand tvp = 1,2-di(4-
pyridyl)ethylene were synthesized, characterized and studied by EPR and magnetic
susceptibility methods. EPR spectroscopy allowed to obtain information on the energy levels
and wave functions of Fe (III), the local symmetry of the ligand field of spin-crossover
complexes, and also on the thermodynamic parameters characterizing the spin-transition
process.
EXPERIMENTAL
The composition of the synthesized compounds were confirmed by the elemental analysis.
Elemental analysis was performed on the EuroVector EA3000 CHNS-O Elemental Analyzer.
Powder X-ray diffraction patterns were recorded on a DRON-7 diffractometer with CuKα
radiation with a β - filter. The EPR spectra were recorded by means of EPR CW spectrometer
Bruker EMXplus (X-band) with temperature console ER4131VT in the temperature range of
100-340 K. The simulation of the obtained EPR spectra was performed by means of the
EasySpin software package.[18] Temperature-dependent magnetic susceptibility was measured
on the equipment (a PPMS-9 magnetometer) of the Federal Shared Facilities Center for
Physicochemical Investigations of Kazan Federal University over a temperature range of 100
to 300 K. All measurements were made on polycrystalline solids.
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Synthesis of the complexes
All the chemicals and solvents used for the syntheses were purified by
recrystallization or distillation and dehydration until their constants coincided with data from
literature. Freshly prepared Schiff bases ligands were used to synthesize Fe (III) complexes.
The ligands were synthesized by mixing equivalent amounts of aldehydes with ethylene
diamine in ethanol, followed by heating with stirring and crystallization while cooling to
room temperature. The products were recrystallized from ethanol twice. During the synthesis,
it was found that, depending on the time of crystallization of the complexes from the solution,
two types of compounds are formed.
[Fe(bzacen)(tvp)]BPh4·2CH3OH·CHCl3 (sample 1). A tvp solution (58.2 mg,
0.32 mmol) was added to a solution of [Fe(bzacen)Cl] (140 mg, 0.32 mmol) in 40 ml of
chloroform/methanol mixture (1:1). This mixture was stirred at 60ºC to dissolve and was
filtered, while hot, into a freshly prepared methanol solution (5 ml) of NaBPh4 (109.1 mg,
0.32 mmol). The resulting mixture was stirred 24 hours at room temperature. Gray-green
fine-crystalline precipitate was filtered, washed with cold methanol and dried in air. Yield:
128 mg, 37%. Calculated: C 67.39; H 5.66; N 5.15 C61H61BFeN4O4Cl3. Found: C 67.48; H
5.55; N 5.06.
[Febzacen(tvp)]BPh4 CH3OH (sample 2) was prepared by the same procedure as the
sample 1, only the stirring time at room temperature was 1 hour. Dark-green fine-crystalline
precipitate was filtered, washed with cold methanol and dried in air. Yield: 20%. Calculated:
C 75.73; H 6.03; N 5.99 C59H56BFeN4O3 Found: C 74.98; H 6.11; N 6.26.
According to the elemental analysis, the composition of complexes 1 and 2 contains a
different number of solvent molecules used in the synthesis in the form of a solvate.
Although synthesis products 1 and 2 were in the form of fine crystalline precipitates,
single crystals of these compounds could not be grown. The powder X-ray diffraction
patterns of the synthesized samples are shown in Fig. 2. The peaks on the X-ray diffraction
pattern of sample 2, indicated by asterisks, are absent on the X-ray diffraction pattern of
sample 1. This shows that the considered compounds have different structures.
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RESULTS AND DISCUSSION
EPR spectroscopy and magnetic susceptibility data
In the EPR spectra of samples 1 and 2 two types of signals were observed. Type A
signals are located in the weak-field part of the spectrum and have effective parameters
geff=4.2 and Heff600 Oe, corresponding to the HS Fe (III) centers (S = 5/2) with a large
value of the fine structure parameter (D>h) and strong rhombic distortions (E/D ~ 1/3). In
the central part of the spectrum, we observed signals of type B. Type B signals have a shape
which is characteristic for the spectrum of orthorhombic symmetry with S = 1/2 and were
assigned to the LS Fe (III) complexes (Fig 3). However, the temperature dependence of the
spectra of samples 1 and 2 is significantly different.
At low temperatures, the HS signal A from the sample 1 is not observed up to ~220 K;
it arises and increases sharply with further temperature rise (Fig. 3a). The temperature
dependence of the integrated intensity of this signal indicates the existence of an incomplete
spin transition in the sample in the temperature range (200-320) K (Fig. 5a). The intensity of
the LS signal B in this temperature range decrease sharply (Fig. 3a). Spectrum B is described
by temperature-independent parameters g1=2.313; g2=2.073; g3=1.947. With increasing
temperature, smoothing of the characteristic peculiarities of the spectrum B was observed (up
to the transition to a single almost isotropic line), as a result of the increase of the relaxation
contribution to the width of the individual line.
The peculiarities of the temperature dependence of the EPR spectra of the sample 2
indicate the existence of a spin transition in it in the temperature range (130 - 230) К. The
integrated intensity of the HS signal A increases, as the temperature rises from ~ 130 K to ~
230 K (Fig. 5b), while the decrease of the integral intensity of the LS signal B is faster than
expected by Curie-Weiss law. Spectrum B is described by temperature independent
parameters g1= 2.355, g2=2.090, g3=1.924, and is also characterized by an increase of the
width of the individual line with temperature increase. The differences in the parameters of
the LS centers for the samples 1 and 2 indicates a change in the properties of the nearest
environment of the Fe (III) ion, which perhaps is due to the different conformation of the
complexes in the samples.
To determine the relative content of HS and LS fractions, decomposition of
experimental spectra into HS and LS components was carried out. An example of modeling
the experimental spectrum of sample 2, measured at a temperature of 190 K, is shown in Fig.
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4. The IHS and ILS values of the integrated intensities of the HS and LS signals were obtained
by double integration of the HS and LS components of the simulated spectrum. The relative
content of the HS centers NHS at a given temperature, NHS was defined as the ratio of the
integral intensity IHS of the HS signal to the sum IHS+ILS of the integral intensities of the HS
and LS signals. The temperature dependence of the normalized integrated intensity of the
EPR signal of the HS complexes for samples 1 and 2 is shown in Fig. 5
The temperature dependence of the effective magnetic moment (expressed in units of
Bohr magnetons) of samples 1 and 2 in the range (100–300) K is presented in Fig. 6. The
value of ca. 2.0B is characteristic of the Fe (III) ions in the LS state, while the value of
5.91 B corresponds to the Fe (III) ions in the HS state. For sample 1 eff(T) show a sharp
increase in the value of eff from 2.53 B at 200 K to 5.12 B at 300 K. This property is due
to the thermally induced incomplete spin transition between HS and LS states in this
temperature range. For sample 2 eff(T) spin-crossover properties are manifested in the lower
temperature range. The plot of the effective magnetic moment vs. temperature of complexes
2 indicated the sharp increase in eff from 3.08 B to 5.33 B with increasing temperature
from 130 K to 230 K. The magnetic susceptibility data are in agreement with the results of
analysis of the temperature dependence of the EPR spectra shown in Fig. 5.
Thermodynamic characteristics of the spin transition
As noted earlier, in the samples 1 and 2 in a certain temperature range, we observed
an anomalous increase of the integrated intensity of the HS signal A for paramagnetic
centers, which is due to existence of the spin transition. The changes in enthalpy and entropy,
causing the spin transition, were determined from analysis of the temperature dependence of
the integral intensity. When spin transition is absent, the integral intensity of the HS isolated
paramagnetic centers obeys the Langeven’s law and in the high-temperature approximation is
inversely proportional to temperature. The population of the HS state also decreases with
increasing temperature according to the Boltzmann's law of the population of energy levels.
At the same time, in the spin transition temperature range, the relative content of the HS
centers of the NHS(T) centers increases with temperature. Given the described processes, a
change in the integral intensity of signal A is represented by the equation
IHS = Iо·NHS (T)·/kT·(1-exp(-Е/kТ))/(1+exp(-Е/kТ)), (1)
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where Е is the energy interval between the ground and excited HS states of the
system, Iо is the normalization factor,  = ħ is the interval between levels, which are
involved in the formation of the EPR signal.
The change of relative content of HS fraction NHS(Т) during spin transition
corresponds to the thermodynamic ratio:[19]
(
)
1 − 푁HS
푁HS − 푟
∆퐻 + Г 1 + 푟 − 2푁HS
∆푆
푅
ln (
) =
−
(2)
푅푇
where  is the change of enthalpy, S is the entropy change, Г is the interaction
parameter, r is the residual HS fraction, which is present at low temperatures in the LS
fraction, R=8.31 JК-1mol-1 is the gas constant.
Satisfactory agreement was obtained when describing the temperature dependence
integrated intensity of signal A of samples 1, 2 by equations (1), (2) (Fig.5а, 5b). The optimal
values of fitting parameters are presented in table 1.
Analysis of the electronic structure of the complexes
The g - factors of the EPR spectra of low-spin Fe (III) complexes observed in samples
1 and 2 were analyzed using the one-electron approximation in the framework of the lower
orbital triplet, similarly to Ref..[16, 20, 21] The coefficients of the wave functions of the lower
triplet A, B, C and the coefficient of suppression of the spin–orbit coupling K were calculated
taking into account the spin-orbit interaction ξ Î ŝ, tetragonal Δ, and orthorhombic V
components of the ligand field. The values of Ei, Δ and V were determined by substituting
the obtained values into the energy matrix and solving the corresponding system of linear
equations; the value of ξ was assumed to be 460 cm–1. The obtained data are given in table 2.
The wave functions of the ground doublet, presented in the basis of real wave
functions, have the form
+
1 = a dyz +> + b dxy –> + c dxz
2 = a dyz –> – b dxy +> – c dxz
>
–
>
(3)
Their coefficients and the obtained values A, B, and C are related by the relations
2
2
a = – i(A+C)/
; b = iB; c= (C – A)/
(4)
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For the obtained values of A, B, C, the coefficient c of dxz> state has a maximum
value in the basis of real wave functions (c=0.995 for sample 1 and c=0.992 for sample 2).
The dxz> state is the ground state of the Fe (III) ion for samples 1 and 2.
The difference of the parameters of the LS centers in samples 1 and 2 indicates a
change of the properties of the nearest environment of the Fe (III) ion.
To obtain a diagram of the energy levels of the [Fe(bzacen)(tvp)]BPh4 complexes
exhibiting spin-crossover properties, the following considerations should be taken into
account. From general thermodynamic considerations, it follows that the main energy level
should be LS, and the level of the HS state should be higher by a relatively small value (of
the order of kT). The results of analysis of g-factors of LS complexes of Fe (III) of the
studied compounds in Table 2 indicate a large value of splitting of the lower orbital triplet
6
(several thousand cm-1). It is possible to neglect the splitting of the multiplet A1 HS in the
crystal field. It does not exceed a few cm-1, since this term has an orbital moment L = 0.
Thus, the HS 6A1 term involved in the spin transition, should be located above the lower level
2
of the split LS T2 term. A similar arrangement of the energy levels participating in the spin
transition was considered in Ref..[16, 22, 23]
The analysis of the experimental g-factors for the LS centers of samples 1 and 2 made
it possible to identify the difference in the magnitude of the low-symmetry distortions of the
ligand field and to determine the corresponding energy level of the lower orbital triplet E1,
E2, E3 (Table 2). Calculations showed that the LS splitting of the orbital triplet is significantly
larger than the magnitude of the “pure spin-orbit splitting” (690 cm-1). In Fig.7, E1, E2 and E3,
indicate the energy levels of the split LS orbital triplet t2g, and EHS indicates the energy of the
excited HS multiplet.
The nearest environment N4O2 of the Fe (III) ion (up to the second coordination
6
sphere) has the same configuration. Therefore, the energy of the term A1g is assumed to be
the same for samples 1 and 2. Then, the decrease in the temperature of the spin transition TC
in sample 2 (208.4 К) compared to 1 (285.8 К) is explained by a decrease in the orbital triplet
splitting for the LS configuration and, accordingly, a decrease in the energy interval EHS-E1
between the main LS and HS energy levels (Fig. 7).
Effect of solvation on the spin-variable properties of Fe (III) complexes
It is well known that in coordination compounds the solvent content has a strong
effect on the spin state of Fe (III) complexes and the parameters of the spin transition.[13, 24, 25]
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As the authors of Ref..[24] note, there are many examples of decreasing the temperature of the
spin transition upon desolvation of a sample or simply the appearance of spin-crossover
properties in previously solvated compounds with only low-spin states. Analyzing data on
recent spin-crossover systems, the authors of Ref..[13] concluded that most often different
solvents modify the behavior of the spin-crossover transition rather than not responsible for
its occurrence. A typical example of such a system is [Fe(qsal-I)2]OTf and [Fe(qsal-
I)2]OTf‧MeOH compounds. X-ray structural analysis and magnetic susceptibility
measurements showed that a loss of solvent leads to a decrease in the spin transition
temperature and the hysteresis width.[26] The authors explain these changes by an increase in
intermolecular interactions due to the formation of additional π-π and hydrogen bonds with
increasing degree of solvation.
Based on the elemental analysis data, the observed differences in the electronic
structure and magnetic resonance properties of the Fe (III) complexes in samples 1 and 2 can
be attributed to structural differences caused by a change in the crystal packing for complexes
with different concentrations of solvent molecules. The temperature of spin transition is
lower in complexes 2, which include a smaller number of solvent molecules. That correlates
with the tendency to decrease the temperature of the spin transition upon desolvation of the
sample described in Ref..[24–26] The powder diffraction patterns shown in Fig. 2 demonstrate
the structural differences between samples 1 and 2.
CONCLUSION
Two types of spin-crossover complexes [Fe(bzacen)(tvp)]BPh4‧nSolv obtained at
different times of crystallization from solution were found. They differ in the local ligand
field characteristics and in the spin-crossover transition parameters. Measurements of the
EPR spectra temperature dependence with their subsequent decomposition into low-spin and
high-spin components made it possible to estimate the relative content of the high-spin and
low-spin fractions and obtain information on the thermodynamic parameters of spin transition
and spin-crossover processes.
The EPR data analysis of compounds with equatorial ligand bzacen led to the
conclusion: if the spin state in highly symmetric Fe (III) complexes is determined by the
2
6
energy interval between the T2 and A1 terms, then in the pseudo-octahedral complexes the
2
splitting of T2 energy levels due to low-symmetry components of the crystal field and spin-
orbit interaction play a significant role in the realization of spin-crossover processes.
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The degree of solvation rather strongly influenced on the electronic structure, on
magnetic and resonance parameters of Fe (III) samples 1 and 2. A decrease of the spin
transition temperature in sample 2 with a smaller number of solvent molecules in the complex
correlates with tendency to decrease the spin transition temperature upon sample desolvation.
Note that the our investigation of two different modifications of spin-crossover
complexes extracted in different time of their crystallization from solution show large
capabilities of the EPR method in studying the kinetics of Fe (III) complexes formation and
their thermodynamic stability.
ACKNOWLEDGEMENTS
The magnetometer studies (R. G. Batulin and M. A. Cherosov) were performed by the
subsidy allocated to Kazan Federal University for the state assignment in the sphere of
scientific activities.
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Table 1.
Thermodynamic characteristics of the complexes [Fe(bzacen)(tvp)]BPh4.
ТС, K
(NHS=NLS)
285.8
,
kJ·mol-1
26.8
S,
,
Compound
r
J·mol-1·K-1 kJ·mol-1
Sample 1
Sample 2
93.9
78.9
0.0
0.0
0.0
208.4
16.4
0.02
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Table 2.
The results of the analysis of the g-factors of LS Fe (III) complexes in the studied
compounds.
Е3,
cm-1
Е1,
cm-1
Е2,
cm-1
,
cm-1
Compound
К
A
B
C
V/
Sample 1
-3299
-2617
-847
-508
4147 -2065 0.194 0.823
3126 -1551 0.221 0.803
0.772
0.782
0.034
0.044
-0.635
-0.621
Sample 2
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FIGURE 1 Schematic representation of the structure for complexes
[Fe(bzacen)(tvp)]BPh4
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FIGURE 2 X-ray powder diffraction patterns of samples 1 and 2
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FIGURE 3 The temperature dependence of the EPR spectra for samples 1 (a) and 2
(b); ν=9.47 GHz
FIGURE 3 The temperature dependence of the EPR spectra for samples 1 (a) and 2
(b); ν=9.47 GHz
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FIGURE 4 Experimental EPR spectrum of sample 2 at 190 K presented as a superposition of
the signals of HS and LS centers with the relative content of HS centers NHS = 0.58
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FIGURE 5 The temperature dependence of normalized integrated intensity of the EPR signal
of HS samples 1 (a) and 2 (b). ■ experiment; ___ calculating using equations (1), (2) with
thermodynamic parameters given in table 1
FIGURE 5 The temperature dependence of normalized integrated intensity of the EPR signal
of HS samples 1 (a) and 2 (b). ■ experiment; ___ calculating using equations (1), (2) with
thermodynamic parameters given in table 1
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FIGURE 6 The temperature dependence of the effective magnetic moment of samples 1 and
2
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FIGURE 7 Supposed energy levels diagram for samples 1 and 2
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Two types of complexes
[Fe(benzacen)(tvp)]BPh4‧nSolv
are synthesized and studied by
Spin
Crossover
Properties
B
of
the
EPR
and
magnetic
The
Fe (III) Complexes
100K
susceptibility methods.
in
130K
thermodynamic parameters of
spin transitions were determined
from the temperature dependence
of the EPR signals integral
intensity. Estimation of the
splitting of the energy levels of
these complexes by analyzing g-
factors of low-spin Fe (III)
centers showed a large role of
low-symmetry deformation of the
[Fe(bzacen)(tvp)]
160K
BPh4‧nSolv Chain
190K
Structures
220K
250K
300K
A
0
4000
8000
T.A.
Ivanova,
H,Oe
I.V.
Ovchinnikov, O.A.
Turanova⁕,
Bazan,
L.V.
ligand
field
in
the
implementation of spin-crossover
processes.
V.A.
Shustov,
R.G. Batulin,
M.A.
Cherosov
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